The invention relates to improvements in photodetector circuits including a photodiode connected to an input of a transimpedance amplifier, and more particularly to improvements thereto which (1) produce both low noise and low input bias current in the input stage of the transimpedance amplifier, (2) avoid nonlinearity due to the presence of a "dead band" caused by offset of the differential input stage, (3) provide an N-type guard tub structure that collects current due to carrier generation caused by infrared or near infrared rays penetrating deeply into the semiconductor material of which the photodiode is fabricated, (4) allow application of an external pull-down voltage to levels below the chip substrate voltage, and (5) provide compensation as a function of transimpedance amplifier output voltage so as to reduce its NEP (noise effective power).
Photodetector circuits are often used in systems wherein high precision, linear low power circuits capable of operating from a single power supply are required. High performance photodetector circuits usually are constructed using a photodiode connected to the input of a transimpedance amplifier. It has been difficult to completely achieve such performance in an integrated photodiode/transimpedance amplifier which must have low input bias current, low input referred noise, is operable from a single power supply, and has an input common mode range which includes ground. FIG. 2 shows a typical prior art transimpedance amplifier of the type commonly used in a photodetector circuit, having a photodiode connected to the (-) input thereof. An input bias current source 40 is connected to the emitter of PNP input transistor 31. A bias circuit is connected to the base of PNP input transistor 32, or else the base of PNP input transistor 32 is connected to ground. In the prior art circuit of FIG. 2, the large bias currents applied by current sources 40 and 41 produce low values of r.sub.e in transistors 31 and 32. That results in low differential noise voltages between the bases of PNP differential amplifier input transistors 10 and 11, so the noise performance of the prior art circuit 2 of FIG. 2 is quite good.
However, bias current sources 40 and 41 of the prior art circuit of FIG. 2 supply large bias currents of 1 to 50 microamperes through PNP input transistors 31 and 32, which results in undesirably high input bias current.
The transimpedance amplifier circuit of FIG. 2 has an undesirable "dead band" of approximately 10 millivolts, which is the value of the typical "worst case" input offset voltage of the differentially coupled input transistors 10 and 11. Consequently, when the transimpedance amplifier of FIG. 2 is used with a photodiode to form a photodetector circuit, it is unresponsive to incident light until the light intensity reaches a level at which photodiode produces enough photocurrent to overcome the "dead band". The associated nonlinearity of the circuit is very undesirable. Nevertheless, the closest prior art single power supply photodetector circuits do not avoid the dead band problem.
An integrated circuit fabricated using a junction-isolated manufacturing process and a P-type substrate typically requires that the P-type substrate be connected to the most negative potential to ensure junction isolation between components fabricated in junction-isolated N-type epitaxial islands, and also to ensure that substrate junctions are never forward biased. In some applications it is desirable to be able to connect a load supplied by V.sub.OUT to a power supply which is more negative than ground, especially in transimpedance amplifiers designed to operate from a single power supply. The use of a pull-down load resistor connected between V.sub.OUT and the negative power supply voltage has the disadvantage that the current through such pull-down load resistor is proportional to the voltage across it. Depending on circuit operation, this can result in excessive power dissipation and also can overload the output NPN transistor 23 of buffer circuit 8 and thereby prevent linear circuit operation in response to the light 6 being detected by photodiode 5.
A typical prior art structure in which a photodiode is fabricated can be understood by referring to a portion of FIG. 3, which shows part of the structure of the present invention. Typically, a lightly doped N-type epitaxial layer is formed on the upper surface of a P-type substrate 50. P+ isolation regions extend through the N-type epitaxial layer to form junction-isolated N-type regions in which isolated transistors, JFETs, and photodiodes can be formed. In a typical bipolar integrated circuit, a junction-isolated N-type epitaxial region functions as the collector of an NPN transistor or as the base of a lateral PNP transistor. In FIG. 3, photodiode 5 includes a shallow P+ ion-implanted region 46 which forms a photo junction with the underlying N.sup.- epitaxial region 51. Light 6 impinging on this structure produces a first photocurrent. P+ contact diffusions 47 are provided to make it convenient to connect the P.sup.+ anode of the photodiode 5 to ground or the (+) input of transimpedance amplifier 2. An N+ buried layer region 44 is provided directly under P+ implanted region 46. An N+ region 48 provides for convenient connection of the cathode of the photodiode to the (-) input of transimpedance amplifier 2.
This much of the structure shown in FIG. 3 is conventional. Infrared or near infrared light 6 may impinge on the entire upper surface of the integrated circuit in which a photodiode, a transimpedance amplifier, and an output buffer are fabricated. Such impinging light 6 may include infrared or near infrared components, indicated by 6B in FIG. 3. Infrared or near infrared light, unlike ultraviolet light, penetrates deeply into the relatively thick P-type substrate 50, producing electron-hole pairs such as 43. Since P-type substrate 50 is lightly doped, these electron-hole pairs have long lifetimes, and therefore form currents, some of which are likely to be "collected" by the various N- epitaxial regions in which the bases of amplifier input transistors (such as 10, 11, 31 and 32 of FIG. 2) are formed, creating undesirable offset voltages. Such electron-hole pairs also may be collected by the N- epitaxial region 51 of photodiode 5, and thereby contribute an "error current" component that is summed with the photocurrents produced by the impinging light 6 to be detected unless something is done to prevent that from occurring.
Prior photodiode structures have included a first "top" photodiode located near the light-receiving surface of the chip and a second "bottom" photodiode having a photo junction located deeper within the semiconductor material. The "top" photodiode of such a structure is responsive to ambient visible light or ultraviolet light, and the "bottom" photo junction is responsive to deeper-penetrating infrared or near infrared light. The photocurrents generated by the two photodiodes have been applied as inputs to separate transimpedance amplifiers.
The "Noise Effective Power" (NEP) is a commonly used parameter for characterizing the performance of an optical detector. By definition, the NEP is the power that the input signal must have in order to produce an output signal that is equal to the RMS noise signal produced in the optical detector circuit. Stated differently, it is the power the input signal must have in order to cause the signal-to-noise ratio of the optical detector circuit to be equal to 1. It would be desirable to provide an optimal photodetector circuit having a lower NEP, which means that the photodetector circuit could detect weaker (i.e., lower amplitude) light signals.
It has been found that the structure of FIG. 3 provides a superior photodiode that is responsive to a much broader range of ambient light wavelengths than the closest prior art of which the inventors are aware, which prior art applies the photocurrents provided by the "top" and "bottom" photodiodes to the inputs of two separate transimpedance amplifiers, respectively.